1. Field of the Invention
The present invention relates to a mobile station which is located by use of cellular communications.
2. Description of the Related Art
JP-A-7-181242 discloses a CDMA-based system in which respective base stations are GPS synchronized in accordance with a cellular scheme. In this system, a mobile station is located by observing a reception timing of a pilot signal transmitted from each base station.
FIG. 3 illustrates exemplary results of correlation calculations. The illustrated graphs, referred to as xe2x80x9cdelayprofiles,xe2x80x9d show which delayed paths are observed. The horizontal axis represents a delay time, i.e., the reception timing which has been compensated for by transmission timing. The delay time is represented in units translated to the number of chips of a spread code. The vertical axis represents the output of the correlation calculation, where a large correlation value indicates reception of signal, i.e., existence of a path. Using these results, it is possible to find a relative delay time of a wave required to propagate from a base station to a mobile station. Here, the delay time is represented by a relative value since the absolute time is unknown at a mobile station. the found relative delay time may be multiplied by the velocity of light to find a pseudo range. When the pseudo ranges can be found for at least three base stations or more, the position of a mobile station can be estimated through triangulation.
A mobile station may estimate the position of a path using a correlation calculating circuit (despreading circuit) for detecting signals from base stations. For further estimating the position of the path in greater details, a delay profile as illustrated in FIG. 3 is created to pick up several points of the phase at which the code exhibits peaks. Then, the points are interpolated to estimate a possible peak which may exist within a sample interval. In this way, the position detection involves the despreading for multiple points on the signal phase. However, a hardware provided with a large number of despreading circuits in parallel would result in an increased scale of the circuit.
On the other hand, the work associated with the position detection is not required to provide an immediate response. Thus, received data maybe once stored in a memory, such that a small number of despreading circuits are repeatedly used to avoid an increase in the circuit scale. However, in a plurality of base stations, signals from remote base stations are weaker than those from nearby base stations, so that the correlation calculation must be performed for a long time period. For storing data over such a long time period, a large amount of memory is required.
The correlation calculation performed for a long time period involves another problem which will be discussed below. Some mobile stations for use in mobile communications may employ a local oscillator which is not so high in frequency accuracy for purposes of reducing the cost, and is synchronized to a nearby base station to reduce a carrier frequency offset (auto frequency control (AFC) function). However, the limited capability of the AFC function causes several Hz of frequency difference to remain between a mobile station and a base station, so that complete synchronization cannot be achieved even if no fading occurs. For this reason, the phase of a received signal presents a slow rotation at several Hz. Thus, even if the user of a mobile station is stationary or is slowly moving at about a walking velocity, a received signal goes rotating, thereby presenting difficulties in performing coherent adding for a long time period.
Here, the coherent adding refers to a process for adding a signal while correcting the phase of the signal. With the coherent adding performed for N codes, desired signal components have the power N2 times as much, while noise power or interference power is N times. Thus, the signal to noise power ratio is improved by a factor of N. However, the rotation of the phase prevents the coherent adding of a desired signal, so that such effect is reduced.
Since a signal from a remote base station equivalently has a reduced S/I (signal to interference power ratio), the number of times of the coherent adding is desirably increased to improve the signal quality. However, due to the foregoing phenomenon of phase rotation, the number of times of the coherent adding cannot be increased beyond a certain limit. For example, with a mobile station which employs a carrier frequency at 800 MHz and presents a frequency stability of 0.01 ppm after AFC, the frequency of the phase rotation is calculated to be 8 Hz. Assuming that the limit of phase rotation required for the coherent adding is within 45 degrees, it is understood that the coherent adding can be performed within 0.015625 seconds (=xe2x85x9xc3x9745/360). If the coherent adding were performed for a time period longer than that, the result would be rotation of a signal vector which causes deteriorated S/I on the contrary.
The above problem is solved by a wireless location handset apparatus for estimating the position of a mobile station using signals coming from at least first and second base stations in a cellular communication system, wherein the amount of phase rotation for a signal received from the first base station is estimated, and the wireless location handset apparatus comprises a memory circuit for storing the received signal compensated for the phase rotation.
A periodic signal is transmitted from a cellular base station. The memory circuit has a capacity for accumulating the signal for one cycle. Newly received information is added to information previously accumulated in the memory circuit, and the resultant information is again accumulated to eliminate the memory capacity exceeding the capacity for one cycle of the signal. It should be noted that since a pilot signal transmitted from a CDMA base station is a non-modulated signal, the same pattern is repeated at PN code cycles of 26.6 milliseconds (ms). In this case, therefore, the memory circuit is required to have only a capacity for storing information for 26.6 ms.
According to the present invention, for locating a mobile station using a cellular communication system, it is possible to reduce the circuit scale and the amount of memory as well as to reduce the number of times of operations required for long-period coherent adding.